1. Field of the Invention
The present invention relates to a pixel structure of a solid-state image sensor, more particularly to the improvement of pixels that constitute a solid-state image sensor that can be used as a time-of-light-flight distance sensor, which measures optical time-of-flight by using an optical time-of-flight measurement method (TOF: Time of flight) by receiving reflection light of light irradiated to an object and measures a distance to the object based on the optical time-of-flight, an image sensor that obtains a three-dimensional image of the object, or the like. Particularly, the invention relates to a pixel structure of a solid-state image sensor that can be used as a pixel of a solid-state image sensor employing a charge sorting method which can be used under unknown background light illumination such as a solid-state image sensor that is equipped with a plurality of charge-storage sections and discriminate photoelectrons generated by incoming light on the timing of incoming light to sort and store the photoelectrons in the plurality of charge-storage sections.
2. Description of the Related Art
Generally, there is known a so-called optical time-of-flight measurement method (TOF: Time of flight) in which intensity-modulated illumination light by pulse or high frequency is irradiated on an object, time-of-flight until the reflection light from the object reaches an image sensor is measured to obtain distance.
As a solid-state image sensor that can be used as an image sensor used in the optical time-of-flight measurement method, there is a sensor employing a charge sorting method, for example, which is equipped with a photoelectric conversion section and a plurality of charge-storage sections, discriminates electrons generated in the photoelectric conversion section by the incoming light on timing of incoming light, and sorts and stores the discriminated electrons in the plurality of charge-storage sections.
Meanwhile, according to the solid-state image sensor employing the charge sorting method, conventionally, the processing of distributing electrons generated in the photoelectric conversion section to a plurality of charge-storage sections in response to incoming timing of light has been realized by applying a high-frequency pulse voltage to gate electrodes installed directly above the oxide film on semiconductor surface.
However, due to multiple reasons such as concentration variation in impurity to be doped in a semiconductor substrate near the gate electrodes, instability of the physical shape of the gate electrodes, damage of the oxide film or variation in the film thickness, even if each gate electrode is equipped with the same structure and the same voltage is applied to each gate electrode, there is generally a fear that passages (channels) of electrons formed on the semiconductor substrate of directly under each gate electrode do not become entirely the same, and there is a fear that channels formed directly under gate electrodes are differently formed between each gate and variation occurs.
If variation occurs in the channels formed directly under each gate electrode, electrons that should be distributed to a plurality of charge-storage sections in the same conditions are partially distributed to a charge-storage section adjacent to a gate electrode having a low threshold value, and a problem was pointed out that electrons generated in the photoelectric conversion section could not be distributed correctly to the charge-storage sections.
Conventionally, to solve the problems regarding the distribution of electrons generated in the photoelectric conversion section, a method is proposed in which a photogate structure is employed in the photoelectric conversion section, is charge residue eliminated by the structure and a potential difference exceeding variation in a threshold value of the gate is generated as disclosed in Japanese Patent Laid-open No. 2005-235893 that was presented as Patent Document 1, for example.
However, it is known that the photoelectric conversion efficiency of the photogate is low to light having long wavelength, and for this reason, a new problem incurred in a solid-state image sensor employing a photogate structure in the photoelectric conversion section that a large obstacle in applying to various applications.
For this reason, in view of the various problems that Prior Art has, the present inventor proposed an invention according to the pixel structure of a solid-state image sensor as patent application No. 2007-181696 (filing date: Jul. 11, 2007) that the pixel structure of solid-state image sensor of charge sorting method which distributes electrons used in an optical time-of-flight measurement method or the like to perform charge storage, in which imbalance in the distribution of electrons caused by the variation in a threshold value voltage of a gate for distributing electrons is eliminated, and a photodiode having sensitivity in long wavelength can be used as a photoelectric conversion element.
The invention that the present inventor proposed by Patent Application No. 2007-181696 is that sorting performance of electrons is improved by constituting the gates for distributing electrons in multistage, and they can be used as pixels of the solid-state image sensor which are used in an optical time-of-flight measurement method or the like, more particularly, by changing the structure of gates adjacent to a plurality of charge-storage sections in which variation in a threshold value affects the distribution of electrons generated in the photoelectric conversion section becomes a problem, the variation in the threshold value is prevented from affecting electron sorting capability.
More specifically, the invention that the present inventor proposed by Patent Application No. 2007-181696 is that a front stage gate that performs readout control of photodiode and a rear stage gate that is positioned on the rear stage of this front stage gate and performs control of distributing electrons to a charge-storage section are provided, and by giving a difference of potential exceeding the threshold value variation that the rear stage gate, which performs control of distributing electrons to the charge-storage section, has by the front stage gate the threshold value variation in the rear stage gate that performs distribution of electrons is prevented from affecting the distribution of electrons to the charge-storage section.
Herein, FIG. 1(a) shows a plane structure explanatory view schematically illustrating a principled plane structure of the pixel structure of the solid-state image sensor according to the second embodiment of the invention proposed by Patent Application No. 2007-181696, and FIG. 1(b) shows a cross-sectional structure explanatory view schematically illustrating a principled cross-sectional structure of FIG. 1(a) along B-B line.
When the pixel structure of a solid-state image sensor 30 according to the second embodiment of the invention proposed by Patent Application No. 2007-181696 which is shown in FIGS. 1(a) (b) is compared with a conventional pixel structure of a solid-state image sensor, this pixel structure 30 is different from the conventional pixel structure on the point that a transfer gate (TG) 14 being a read-out gate that is a first-stage gate (front stage gate), which performs readout control of a photodiode (PD) 13 that receives light and generates electrons by photoelectric conversion, and distribution gates (DG) (in pixel structure 30, two distribution gates of a first distribution gate (DG1) 15 and a second distribution gate (DG2) 16 are provided as distribution gates) being third-stage gates (rear stage gate) that are positioned on the rear stage of the transfer gate 14 (hereinafter, should be referred to as “read-out gate 14”) and perform control of distributing electrons to charge-storage sections (FD) (in the pixel structure 30, two charge-storage sections of a first charge-storage section (FD1) 17 and a second charge-storage section (FD2) 18 are provided as the charge-storage sections) are adjacently disposed, and a sub-transfer gate (SG) 31 being movement gate, which is the second-stage gate, is disposed between the read-out gate 14 being the first-stage gate, and the first distribution gate 15 and the second distribution gate 16 being the third-stage gates as a middle gate.
More specifically, the photodiode 13 that takes a wide area in the pixel structure 30 and the read-out gate 14 being the first-stage gate are adjacently arranged, the first distribution gate 15 and the second distribution gate 16 that are a plurality (two in this embodiment) of third-stage gates are adjacently arranged to the read-out gate 14 with a sub-transfer gate 31 being the second-stage gate (hereinafter, should be referred to as “movement gate 31”), and a plurality (which is two because distribution gates are two in this embodiment) of the first charge-storage section 17 and the second charge-storage section 18 corresponding to each distribution gate are respectively arranged adjacent to the first distribution gate 15 and the second distribution gate 16.
Herein, it is preferable that a gap g2 between the read-out gate 14 and the movement gate 31 adjacent to the gate and a gap g3 between the movement gate 31 and plural numbers of the distribution gates (the first distribution gate (DG1) 15 and the second distribution gate (DG2)) adjacent to the gate be as small as possible, and for example, in the case where these gates are gates formed in a polysilicon layer of the same layer, it is preferable to set the gap g2 and the gap g3 to about 0.2 to 0.3 μm that is a minimum value restricted by manufacturing rules, for example.
On the other hand, in the case where these gates are gates formed in different polysilicon layers, the minimum gap created by interface between different polysilicon layers automatically satisfies the conditions of the gap g2 and the gap g3. For this reason, the read-out gate 14 and the movement gate 31 can be constituted so as to have a slightly overlapped region, and the movement gate 31, the first distribution gate 15 and the second distribution gate 16 can be constituted so as to have a slightly overlapped region.
Further, the periphery of each element of the photodiode 13, the read-out gate 14, the movement gate 31, the first distribution gate 15, the second distribution gate 16, the first charge-storage section 17 and the second charge-storage section 18 is separated from a substrate (SUB) 11 by a separating trench (STI) 12.
Next, referring to FIG. 1(b), description will be made for a case where a P type semiconductor substrate is used as the substrate 11 and a solid-state image sensor of the pixel structure 30 is constituted on the substrate 11 composed of this P type semiconductor substrate.
More specifically, in this case, the photodiode 13 is formed by doping low-concentration N type impurity, and the photodiode is formed so as to expand sensitivity on a long wavelength side by making an ion implantation depth rather thick.
Further, the first charge-storage section 17 and the second charge-storage section 18 are drains formed by doping high-concentration N type impurity.
Herein, the doping of N type impurity can be generally formed by a so-called self-alignment method in which ion implantation is performed after forming silicon oxide film (SOX) 19, the polysilicon read-out gate 14, the movement gate 31, the first distribution gate 15 and the second distribution gate 16 on the substrate 11.
At this point, it is necessary to mask a gap between the read-out gate 14, the movement gate 31, the first distribution gate 15 and the second distribution gate 16 to prevent the gap from being doped by N type impurity.
As a result, the gap between the read-out gate 14, the movement gate 31, the first distribution gate 15 and the second distribution gate 16 is formed in the state of the P type semiconductor substrate as it is, that is, to be a native channel.
Herein, a reason why the gap between the read-out gate 14, the movement gate 31, the first distribution gate 15 and the second distribution gate 16 is formed as the native channel is as follows.
More specifically, in the pixel structure 30, it is necessary that potential of the gap between each gate be controlled by a voltage applied to an adjacent gate and a channel of continuous potential be formed, and its simplest method is forming it as the native channel. However, if the potential of a gap between each gate is controlled by a voltage applied to an adjacent gate and the channel of continuous potential is formed, it goes without saying that appropriate impurity may be doped in the gap between each gate.
It is to be noted that each element of the photodiode 13, the read-out gate 14, the movement gate 31, the first distribution gate 15, the second distribution gate 16, the first charge-storage section 17 and the second charge-storage section 18, as described above referring to FIGS. 1(a) (b), is generally formed on the P type semiconductor substrate, but not limited to the P type semiconductor substrate and it goes without saying that it may be formed on a P well (PW) that is formed by doping P type impurity at middle concentration.
Further, it goes without saying that an N type semiconductor substrate may be used as the substrate 11 and all the P type and the N type of each element are inverted as described above, and each element may be formed on the N type semiconductor substrate, and similarly, each element may be formed on an N well.
Although an appropriate size can be arbitrarily selected for the size of the solid-state image sensor in response to an object or an application, a square having a length L of one side at about 10 μm to 75 μm, more specifically, about 10 μm squares to about 75 μm squares is practical as the entire size of the pixel 30.
Further, the gate length of each gate should be optimized based on substrate impurity concentration, oxide film thickness, and conformity of forming channel by an applied voltage, and about 0.5 μm to 1.5 μm is appropriate, for example.
Next, referring to FIGS. 2(a) (b) (c) (d), description will be made for potential in semiconductor and the movement of electrons generated in the photodiode 13.
FIGS. 2(a) (b) (c) (d) are explanatory views schematically illustrating potential in semiconductor corresponding to each element in the cross-sectional explanatory view shown in FIG. 1(b) when a voltage is applied to the read-out gate 14, the movement gate 31, the first distribution gate 15 and the second distribution gate 16.
It is to be noted that square figures illustrated directly above the potential in FIGS. 2(a) (b) (c) (d) show the read-out gate 14 for “TG”, the movement gate 31 for “SG”, the distribution gate (means either one of the first distribution gate 15 or the second distribution gate 16) for “DG”, in which a state where the square is in outline shows that electric potential near substrate potential Vss is given, and on the other hand, a state where the square is painted out in black shows that positive potential is applied.
In the three-stage gate structure by the pixel structure 30, in which the movement gate 31 being the second-stage gate is provided between the read-out gate 14 being the first-stage gate and the distribution gates being the third-stage gates (the first distribution gate 15 and the second distribution gate), a voltage near substrate potential Vss or a voltage Vtg is applied to the read-out gate 14, the voltage near the substrate potential Vss or a power source voltage Vdd is applied to the movement gate 31, and the voltage near the substrate potential Vss or a voltage Vdg is applied to the distribution gate (DG) (the first distribution gate 15 or the second distribution gate 16).
It is preferable to optimize the voltage Vtg and the voltage Vdg as described below.
More specifically, since a voltage creating sufficient potential gradient to the power source voltage Vdd is desirable as the voltage Vtg, about ½ the power source voltage Vdd is acceptable. Further, it is desirable that the voltage Vdg be near the substrate potential Vss because its voltage difference from the power source voltage Vdd is proportional to the number of storable electrons, and about ⅓ the power source voltage Vdd is desirable because potential gradient in moving electrons should also be taken in consideration.
Specifically, it is preferable to set the voltage Vtg and the voltage Vdg to 1.8V and 1.0V respectively, for example, when the power source voltage Vdd is 3.3V.
Herein, FIGS. 2(a) (b) (c) (d) show methods of moving electrons in the four states shown in FIG. 2(a), FIG. 2(b), FIG. 2(c) and FIG. 2(d) by potential.
It is to be noted that the four states are a basic state (a state of electron storage) shown in FIG. 2(a), a state of electron transfer shown in FIG. 2(b), a state of separating valley of potential shown in FIG. 2(c), and a state of re-transferring electrons shown in FIG. 2(d) as described below.
More specifically, the basic state is the state of electron storage being a state where electric potential near the substrate potential Vss is given to the read-out gate 14, the movement gate 31 and the distribution gates, the photodiode 13 is reset to the voltage Vtg, and the charge-storage section is reset to the power source voltage Vdd. FIG. 2(a) shows this basic state (state of electron storage).
When exposed to light in the basic state shown in this FIG. 2(a), photoelectrons are stored in the photodiode 13 and the potential of the photodiode 13 slightly rises.
Next, when the voltage Vtg is applied to the read-out gate 14 and the power source voltage Vdd is applied to the movement gate 31, potential directly under the read-out gate 14 and the movement gate 31 is pushed down, and electrons move to a valley of potential created directly under the movement gate 31 (state of electron transfer) as shown in FIG. 2(b).
Next, when electric potential near the substrate potential Vss is given to the read-out gate 14 to return the voltage, the power source voltage Vdd is kept applied to the movement gate 31, and the voltage Vdg is applied to the distribution gates, a wall of potential is created directly under the read-out gate 14 and the movement gate 31 is separated from the valley of potential created directly under the photodiode 13 (the state of separating valley of potential) as shown in FIG. 2(c). Meanwhile, at this point, electrons cannot go out of the valley of potential directly under the movement gate 31.
Next, when the voltage Vdg is kept applied to the distribution gates and electric potential near the substrate potential Vss is given to the movement gate 31 to return the voltage, the valley of potential directly under the movement gate 31 disappears as shown in FIG. 2(d), so that the electrons that existed in the valley move to a channel on the distribution gate side having low potential, electrons that moved to a channel directly under the distribution gate (DG) further move to the charge-storage section having lower potential without staying, and stored in the charge-storage section (the state of re-transferring electrons).
As described above, according to the pixel structure 30 equipped with three stages of gates, electrons can be completely transferred from the photodiode 13 to the charge-storage section.
Meanwhile, the present inventor prototyped a solid-state image sensor of a pixel structure which is equipped with the pixel structure 30 having the gate structure of three stages and devised so as to prevent variation in a threshold value of the rear stage gates performing distribution of electrons from affecting distribution of electrons to the charge-storage section, conducted various experiment, and confirmed its effect.
As a result, the inventor could confirm that the pixel structure 30 had the effect of suppressing variation.
On the other hand, the inventor also simultaneously confirmed that there remained problems to be solved in achieving high-speed charge sorting in the pixel structure 30. The problems are specifically Problems 1 to 3 as described below.
Hereinafter, description will be made for Problems 1 to 3 of the pixel structure 30, and to make the Problems 1 to 3 easily understood, the operation of the pixel structure 30 will be described again referring to a principled plane structure explanatory view shown in FIG. 3 which schematically illustrates polysilicon regions (regions shown by solid line in FIG. 3) and an active region (region shown by dashed line in FIG. 3) in the pixel structure 30 in a sectionalized manner (plane structure explanatory view shown in FIG. 3 is a plane structure explanatory view corresponding to FIG. 1(a)), and a principled cross-sectional structure explanatory view shown in FIG. 4 schematically illustrates the regions of N type or P type in the pixel structure 30 in detail (the cross-sectional structure explanatory view shown in FIG. 4 is a cross-sectional structure explanatory view corresponding to FIG. 1(b)).
Meanwhile, in the pixel structure shown in FIG. 3 and FIG. 4, regarding constitution equal or corresponding to the constitution shown in FIGS. 1(a) (b), detailed description of the constitution and function should be appropriately omitted by showing in the same numerical characters as the numerical characters used in FIGS. 1(a) (b).
Further, although not shown in FIG. 3 and FIG. 4, areas other than the photodiode 13 of the pixel structure 30 are blocked by metal wiring layer or the like in the same manner as shown in FIGS. 1(a) (b) and light is irradiated only to the photodiode 13 of the pixel structure 30.
Then, as described referring to FIG. 2, a positive voltage as shown in FIG. 5 should be applied in order to each gate of the read-out gate 14, the movement gate 31, the first distribution gate 15 and the second distribution gate 16, and its repetition frequency is 1 to 30 MHz.
Meanwhile, it is preferable that the photodiode 13 be formed by a pinned structure similar to the one disclosed in Japanese Patent Laid-open No. 2007-110162.
Further, boundary of each region such as boundary between the N type region and P type region shown in FIG. 4 is schematically expressed for easy understanding, and there is actually distribution of impurity concentration depending on a semiconductor process.
Herein, regarding dope concentration of impurity in each region, it is preferable to set the impurity concentration of the region of “PW” (P type impurity middle-concentration doped region) higher by about a single digit than the impurity concentration of a substrate (epilayer) composed of the region of “P−” (P type impurity low-concentration doped region), the impurity concentration of the region of “N−” (N type impurity low-concentration doped region) higher by about a single digit than the PW region, and the impurity concentration of the region of “N+” (N type impurity high-concentration doped region) higher by a single digit or more than the region of N−. It is to be noted that ionic species to be implanted should only be appropriately selected.
Further, it is preferable that the level (size) of layout or a range of thickness of each layer be as shown below (refer to FIG. 3 and FIG. 4).
More specifically, it is preferable that the size of layout in planar view be as follows.
Dimension a: 1.5 to 10.5 μm
Dimension b: 1.5 to 10.5 μm
Dimension c: 1.2 to 8.4 μm
Dimension d: 0.35 to 1.8 μm
Dimension e: 0.25 to 1.5 μm
Dimension f: 0.25 to 1.5 μm
Dimension g: 0.5 to 1.5 μm
Dimension h: 0.5 to 1.5 μm
Dimension i: 0.5 to 1.5 μm
Gap g2: 0.1 to 0.3 μm
Gap g3: 0.1 to 0.3 μm
Gap g4: 0.1 to 0.3 μm
Gap g5: 0.1 to 0.3 μm
Gap g6: 0.25 to 0.75 μm
Further, the thickness of each layer depends on a process used in manufacturing, and thickness below is preferable.
More specifically, the thickness below is preferable.
Thickness j (P type substrate (epitaxial layer)): 5 to 22.5 μm
Thickness k (P well layer): 4 to 13.5 μm
Thickness l (photodiode region): 1 to 6.8 μm
Thickness m (charge-storage section): 0.1 to 0.8 μm
Thickness n (P well layer): 2 to 13.5 μm
Thickness o (SOX thickness): 2.5 to 22.5 nm (depends on a working voltage)
Thickness p (gate thickness): 0.05 to 0.5 μm
Herein, when light is irradiated on the pixel structure 30, the irradiated light is ionized into electrons and holes by depletion layer that spreads in a junction area between the N type impurity low-concentration doped region of the photodiode 13 and the substrate 11 being the P type impurity low-concentration doped region.
Then, the ionized holes are absorbed by the substrate 11, and the electrons are stored in the N type impurity low-concentration doped region of the photodiode 13 (refer to FIG. 2(a)).
Then, when a positive voltage is applied to the read-out gate 14 being the first-stage gate after certain time passed, the electrons stored in the N type impurity low-concentration doped region of the photodiode 13 move to the channel formed directly under the read-out gate 14.
Next, when a positive voltage is applied to the movement gate 31 being the second-stage gate, the electrons can be moved to the channel formed directly under the movement gate 31 (refer to FIG. 2(b)).
Then, when the applied voltage of the read-out gate 14 is returned to zero after short time passed, the electrons that exist in the channel directly under the read-out gate 14 are pushed out to the channel directly under the movement gate 31 to which a voltage is applied or the photodiode 13.
Herein, in the case where the potential of the movement gate 31 is lower than potential at which electrons return to the photodiode 13, all electrons move directly under the movement gate 31 (refer to FIG. 2(c)).
Next, when a positive voltage is applied to either one of the first distribution gate 15 and the second distribution gate 16 as a pair of distribution gates being the third-stage gates, electrons move to the channel directly under the first distribution gate 15 or the second distribution gate 16 to which a voltage was applied.
Herein, when the applied voltage of the movement gate 31 is returned to zero, the channel directly under the gate disappears, and electrons completely move to the channel created directly under the first distribution gate 15 or the second distribution gate 16 to which a voltage was applied (refer to FIG. 2(d)).
Next, when the applied voltage of the first distribution gate 15 or the second distribution gate 16 to which a positive voltage was applied is returned to zero, electrons that exist in the channel directly under the gate completely move to the first charge-storage section 17 or the second charge-storage section 18, which is formed of the N type impurity high-concentration doped region, and all electrons are collected.
Herein, by alternately applying a voltage to the first distribution gate 15 and the second distribution gate, electrons generated by light, which was irradiated synchronously with the series of gate-applied voltages, can be sorted to the first charge-storage section 17 and the second charge-storage section 18 being two charge-storage sections.
FIG. 6 schematically illustrates the series of electron movement. Description will be made while referring to FIG. 6, in which electrons move through the photodiode 13 (refer to arrow A), pass through the read-out gate 14 being the first-stage gate based on a voltage applied to each gate (refer to arrow B), and pass through the movement gate 31 being the second-stage gate (refer to arrow C).
It is to be noted that arrow C is drawn slightly above the center of the movement gate 31 in FIG. 6, but areas where electrons actually pass are expected to be distributed stochastically, and a case of such distribution is drawn here.
Then, since a voltage is applied to either one of the first distribution gate 15 and the second distribution gate 16 which are the third-stage gates, electrons pass through either one distribution gate in a route of arrow D or arrow E in response to the application of voltage to the first distribution gate 15 and the second distribution gate 16.
It is to be noted that the gaps (g2, g3) between each gate should be the closest distance that is permitted in a manufacturing process as described above.
Although potential by the gate-applied voltage does not seem to be applied to these gaps (g2, g3), a slight voltage is actually applied due to the lateral effect of gate to form a shallow channel, and electrons can be moved.
However, as the present inventor prototyped the solid-state image sensor equipped with the pixel structure 30 and conducted various experiment as described above, the inventor discovered Problems 1 to 3 described below.
(1) Problem 1
As it is self-evident when seeing the schematic view of electrons movement shown in FIG. 6, the electron-moving distance in the photodiode 13 (refer to arrow A) is longer than the moving distance of electrons when passing through each gate (refer to arrow B to E).
However, force that attracts electrons in the photodiode 13 is generated by a voltage applied to the read-out gate 14 being the first-stage gate that is positioned at one end portion being one end of the photodiode 13.
Therefore, attraction that electrons, which were generated near an end portion on the opposite side of a side of the photodiode 13 where the read-out gate 14 is positioned, receive is weaker than attraction that electrons, which were generated near the side of the photodiode 13 where the read-out gate 14 is positioned.
Herein, according to the consideration of the present inventor, if the photodiode 13 is an ideal photodiode as shown in FIG. 7(a), electrons are expected to be accelerated to reach the channel directly under the read-out gate 14 in sufficiently short time (a few tens of nanoseconds, for example) according to gradual potential gradient that is generated by a voltage applied to the read-out gate 14 being the first-stage gate.
However, the present inventor considered that unevenness actually occurred in potential generated by a voltage, which was applied to the read-out gate 14 being the first-stage gate, due to non-uniform manufacture of the photodiode 13 and electrons were trapped in the photodiode 13 to become residual as shown in FIG. 7(b)
Then, since it is impossible to predict at which point the electrons residual in the photodiode 13 in this manner move, there is a possibility that read-out by strict time of a nanosecond unit cannot be performed.
In a general image sensor having sufficiently long (a few milliseconds, for example) exposure time comparing to electron moving time, this residual time is negligibly short and the number of residual electrons is negligibly few.
However, when the case where the solid-state image sensor is used as an optical time-of-flight distance sensor is considered, the fact that exposure time in detecting optical time-of-flight is as short as 50 to 100 nanoseconds and the number of electrons generated by incoming light in the time is as very few as a few is taken in account, residual time of electrons and the number of residual electrons caused by the concave of potential in the photodiode 13 become non-negligible.
More specifically, the fear that residual electrons exist in the photodiode 13 is Problem 1 to be solved.
(2) Problem 2
Electrons that were moved from the channel directly under the read-out gate 14 being the first-stage gate and exist in the channel directly under the movement gate 31 being the second-stage gate move directly under either one gate of the first distribution gate 15 and the second distribution gate 16 which are the third-stage gates to which voltage is applied next.
At this point, electrons near the center of the movement gate 31 being the second-stage gate are attracted to the third-stage gate to which a voltage was applied (the first distribution gate 15 or the second distribution gate 16) and correct distributed.
However, an electron at the end portion of the movement gate 31 being the second-stage gate in the vertical directions on FIG. 6, that is, the electron shown by numerical character F which is at the end portion on the upper side of the movement gate 31 in FIG. 6, for example, has different routes to be moved in the case of moving to the first distribution gate 15 being the third-stage gate closer to electron F as shown in arrow D and the case of moving to the second distribution gate 16 being the third-stage gate farther from electron F as shown in arrow E.
Herein, when electron F moves to the first distribution gate 15 that is the closer third-stage gate as shown in arrow D, electron F receives strong attraction by potential caused by a voltage applied to the first distribution gate 15 that is the third-stage gate, so that electron F securely moves to the first distribution gate 15.
On the other hand, in the case where electron F needs to move to the second distribution gate 16 that is the farther third-stage gate as shown in arrow E, attraction that electron F receives becomes weaker due to a long distance.
Thus, even if attraction that electron F receives is weak, moving according to weak attraction generated by a small potential difference is made possible if the channel directly under the movement gate 31 being the second-stage gate has an ideally uniform structure.
However, unevenness of potential actually exists in the channel formed directly under the movement gate 31 being the second-stage gate due to manufactural non-uniformity as described in Problem 1, electrons cannot come out from the potential concave that exist in a course, and a case where electrons have difficulty of moving to the farther distribution gate is considered.
Then, in such a case, electrons become residual for short time directly under the movement gate 31 being the second-stage gate or a gap between the movement gate 31 being the second-stage gate and the first distribution gate 15 or the second distribution gate 16 which is the third-stage gate.
As described, when a distribution gate to which a voltage is applied is changed in the distribution gate being the third-stage gate (the first distribution gate 15 or the second distribution gate 16) and a voltage is applied to the distribution gate (the first distribution gate 15 or the second distribution gate 16) being the third-stage gate closer to the residual electrons while electrons are residual directly under the movement gate 31 being the second-stage gate or the gap between the movement gate 31 being the second-stage gate and the first distribution gate 15 or the second distribution gate 16 which is the third-stage gate, strong potential gradient makes the residual electrons move to a channel directly under the closer distribution gate.
More specifically, a destination of electrons is decided depending on where in the channel directly under the movement gate 31 being the second-stage gate electrons exist, and even when a voltage is alternately applied to the first distribution gate 15 and the second distribution gate 16 which are the third-stage gates aiming at distribution by time, there is a fear of a phenomenon that electrons do not pass through a distribution gate farther than a position where electrons exist in the channel directly under the movement gate 31 being the second-stage gate occurs.
More specifically, a fear that electron transfer from the movement gate 31 being the second-stage gate to the distribution gates being the third-stage gates (the first distribution gate 15 and the second distribution gate 16) becomes uncertain is Problem 2 to be solved.
(3) Problem 3
The pixel structure 30 of the solid-state image sensor shown in FIG. 3 and FIG. 4 is manufactured by a multistage integrated circuit process in which a plurality of masks are used.
In the manufacturing process by such an integrated circuit process, it is impossible to eliminate an error of a mask itself and a positioning error.
In short, even when pixel structure 30 of the solid-state image sensor is manufactured aiming at a uniform structure, occurrence of slight variation or imbalance must be accepted.
Herein, when imbalance of a threshold value occurs in the first distribution gate 15 and the second distribution gate 16 which are the third-stage gates, a difference occurs between the electron transfer efficiency of one distribution gate and the electron transfer efficiency of the other distribution gate.
Further, if a small capacitance difference is between the first charge-storage section 17 and the second charge-storage section 18 being two charge-storage sections, a difference of charge-voltage conversion efficiency occurs between the both.
Although such a problem is also a general problem in an integrated circuit, it could become a capital problem due to the fact that the number of electrons dealt with is extremely small when the case of using the solid-state image sensor as an optical time-of-flight distance sensor is taken in consideration.
More specifically, occurrence of imbalance in electron transfer efficiency by manufacturing accuracy (error) is Problem 3 to be solved.
[Patent Document 1]
    Japanese Patent Laid-open No. 2005-235893